High speed broadband networks have been growing rapidly in the last few decades. With the advance of wireless communications technologies, new broadband wireless access (BWA) networks offering 100 Megabits per second (Mbps) to 1 Gigabits per second (Gbps) data rates are on the horizon. The emerging BWA networks pose major challenges to backhaul; that is, the connection of the wireless access points for users of the BWA network to a larger wired network (such as the Internet). In countries with large geographical areas of low population density, such as Australia, high data rate wireless backhaul is necessary to bring broadband services to remote areas economically.
One of the major challenges in wireless backhaul is to achieve both high data rates and long distance. The former demands a large bandwidth in the order of GHz, and the latter requires that the system operate at microwave frequencies rather than the higher millimeter-wave frequencies. Whilst a large contiguous bandwidth is rarely available at microwave frequencies for use in wireless backhaul, there are usually some disjoint bands and sub-bands available. For example, in Australia, such bands are presently available at 6, 6.7, 8, and 11 GHz. These bands may be aggregated to increase transmission capacity and thus obtain the required GHz bandwidth. Link aggregation, or striping, was originally employed for wired digital circuit aggregation, and was soon integrated in ATM, Ethernet, IP, and TCP networks. More recently, there has been interest in wireless striping.
One requirement of striping is that it be transparent to upper communication layers in the Open Systems Interconnection (OSI) model. Preservation of data order is one of the most important transparency requirements. However, when a traffic stream is striped across multiple links, its data may be received out of order at the receiver because of different delays in different stripe links. Most previous approaches deal with out-of-order arrivals by numbering the data packets at the transmitter node with a sequence number. This sequence number can then be used at the receiver node to sort the packets back into order. However, such reordering significantly increases the upper bound on the overall striping delay. Additionally, data packets need to be modified to carry sequence numbers, and a reordering buffer has to be used to store out-of-order packets, significantly increasing cost. Moreover, in certain applications, sequence numbers cannot be added due to packet size or hardware restrictions.
In-sequence striping, or FIFO delivery, can avoid the costs of reordering. However, existing FIFO delivery techniques are unable to achieve “work-conserving”. Work-conserving means that no link is idle when there is data that could be sent through that link. Since link aggregation is often used to overcome communication bottlenecks, it is advantageous that all the links are fully utilized to maximize the throughput and minimize the delay through the aggregated channel.
There are additional difficulties for implementing striping in multi-band wireless backhaul systems. First, the band data rate may be different (heterogeneous) for different bands depending on the available spectrum bandwidth on that band. Second, the band data rate may be time varying, since the modulation and coding level in a band is often made adaptive to time-varying channel quality.